1. Field of the Invention
The present invention relates to a driving unit including a combination of a shaft and helical gears and an image forming apparatus using the same.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a laser scanner includes a laser scanning unit for emitting a laser beam onto the peripheral surface of a photoconductive drum. A slit-like laser emission aperture of the laser scanning unit is normally covered by a transparent plate such as a glass plate. However, if this transparent plate is stained due to deposition of toner, image quality may be affected. Accordingly, to regularly and automatically clean the transparent plate, it has been proposed to provide the laser scanning unit with a wiping member for wiping off stains such as toner attached to the surface of the transparent plate and a driving unit for reciprocating the wiping member along a longitudinal direction of the transparent plate.
A structure for transmitting a drive force of a motor to a plurality of spur gears via a shaft and further transmitting it to the wiping member via the spur gears has been conventionally known as the driving unit for reciprocating such a wiping member.
A mechanism using helical gears in place of spur gears in such a driving unit, has been studied and developed in recent years by the inventors of the present application. The driving unit using the helical gears includes a plurality of first helical gears connected to a shaft and a plurality of second helical gears perpendicular to and engaged with the first helical gears. In this driving unit, the first helical gears rotate together with the shaft when the shaft is rotated by a motor. Further, the second helical gears perpendicular to and engaged with the first helical gears also respectively rotate. In this way, the wiping members connected to the second helical gears can be driven.
In a driving unit including normal helical gears, a drive shaft is rotated only in one direction. Thus, a thrust plate for positioning helical gears coaxially fixed to the drive shaft in an axial direction (i.e. thrust direction) supports only one surface of each helical gear.
However, in the driving unit for cleaning as described above, the shaft, the first helical gears and the second helical gears need to be driven and rotated in both forward and reverse directions to drive the wiping members for cleaning in both forward and reverse directions. When being driven and rotated in both forward and reverse directions, the first helical gears receive forces acting in directions (thrust directions) parallel to the axial direction of the second helical gears (hereinafter, referred to as thrust forces) as reaction forces from the second helical gears in both forward and reverse directions from the second helical gears.
Here, in the case of fabricating members for holding the first helical gears and the second helical gears, it is necessary to form accommodating portions which are open in either one of the thrust directions of the second helical gears in view of assemblability of the driving unit.
In such holding members with one open side, when the shaft and the helical gears are rotated in one of forward and reverse directions by the motor, a thrust force acting on the shaft and the first helical gears in a direction parallel to the thrust direction of the second helical gears can be received in the accommodating portions of the holding members. However, a thrust force acting in an opposite direction when the shaft and the helical gears are rotated in the other rotational direction is facing toward the open sides of the accommodating portions and, hence, cannot be received by the accommodating portions of the holding members. Thus, it becomes difficult to position the shaft and the first helical gears at predetermined positions.